[llvm] [llvm-ar] Use COFF archive format for COFF targets. (PR #82642)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

In theory, the behavior difference between llvm-ar and native "ar" could cause issues in edge cases.  In practice, I've never heard of it causing anyone issues. Build systems generally don't create empty archives, or stick random files into archives.  And even if they do, it only matters if you try to mix llvm-ar with a non-LLVM native archiver/linker; lld can parse any archive format.

https://github.com/llvm/llvm-project/pull/82642